The present invention relates to radio communication systems including CDMA cellular and PCS telephone networks, and in particular, to a method and apparatus for testing such networks with the measurement results corrected for the distortions caused by interference from adjacent and coincident channel emissions and other interference.
CDMA cellular and PCS telephone networks are a type of mobile communication network that use pseudo noise digital codes (xe2x80x9cpn-codesxe2x80x9d). Pn-codes spread the transmitting signals"" spectrum so that multiple communications can occur over the same broadband transmission. Multiplexing and demultiplexing a particular communication signal within the common frequency allows multiple many communications to occur simultaneously.
Testing devices, including pn-scanners, assess the adequacy of signal coverage within a service area for cellular and PCS network. These devices also measure the quality of the CDMA signals from a base station. One measurement device is the pilot scan, measured at a reception site.
The pilot scan provides a graph of base station pilot signal""s strength relative to the initial sequence offset. The mathematical representation is Ec/Io where Ec (energy per chip) is the relative strength of the signal relevant to the communication and Io is the power of the entire transmission signal within the bandwidth. This Ec/Io measurement is in decibels.
The pn-scan measures the quality of the signal received as indicated by its relative composition (how strong is the pilot component in reference to other code components, such as paging, synchronization, and traffic channels) and by modulation and synchronization quality. For example, if the base station Ec/Io measurement deviates from the recommended standards (EIA/TIA IS-97 for the US), or if the carrier is not sufficiently stable, the result will show a value of Ec/Io that is outside the recommended value.
One can distinguish between two measurement goals, which may exist simultaneously, or one at a time: 1) to assess the quality of the signal received at the test site from the standpoint of its fitness for good communication; and 2) to assess the correctness of the transmitter adjustment and modulation quality. Deviation beyond a quality limit alerts the tester to a potential problem that may be caused by a propagation obstacle or by equipment problems at the cell site.
Interference caused by emissions of CDMA (spread-spectrum) and discrete FDMA signals at adjacent and coincident-channel frequencies, as well as in-band, will distort the measurement results. The distortion will be caused in most cases by the increased overall power in the bandwidth of the measurement instrument. This increase of the Io value will in its turn cause a commensurate decrease in the Ec/Io value. Although this decreased value of the received Ec/Io may correctly reflect the reception conditions in the service area in many cases, it does little to help identify the sources of the problem or to verify the correctness of the base station operation.
Various methods for testing in a code division multiple access (CDMA) spread spectrum environment have been the subject of invention. For example, it has been proposed to creating simulated signal interference in a communication channel within a communication system and estimating the average data rates for composite signal energy. For details, refer to U.S. Pat. No. 5,596,570 issued to Soliman for a xe2x80x9cSystem And Method For Simulating Interference Received By Subscriber Units In A Spread Spectrum Communication Network.xe2x80x9d Another technique for simulating signal interference for various testing purposes is discussed in U.S. Pat. No. 5,675,581 issued to Soliman for a xe2x80x9cSimulating User Interference In A Spread Spectrum Communication Network.xe2x80x9d
It has been proposed to optimize the quality of a received signal in a radio receiver by using a novel gain controller for adjusting the gain of the radio receiver to enhance quality. For details, refer to U.S. Pat. No. 5,758,271 issued to Rich et al. for an xe2x80x9cApparatus And Method For Optimizing The Quality Of A Received Signal In A Radio Receiver.xe2x80x9d
A method has been described for establishing a radio subscriber unit including switched antennas in controlling a diversity receiver apparatus in a radio subscriber unit. The system receives a signal line comprising an Ec-Io ratio and determines if a predetermined threshold, which is set to maintain a desired frame error rate, is achieved. Thus quality in the form of the Ec-Io ratio is monitored in the invention. For details, refer to U.S. Pat. No. 6,023,615 issued to Bruckert et al. for a xe2x80x9cMethod for controlling a diversity receiver apparatus in a radio subscriber unit.xe2x80x9d
It has been proposed to calculate signal power using remodulation based upon provided carrier signals. A calculation for interference power uses a difference between a received signal and remodulated signal. Depending upon the results of measurements, power may be controlled to the system. For details, refer to U.S. Pat. No. 6,070,086 issued to Dobrica for a xe2x80x9cClosed Loop Power Transmitter Power Control Unit For A CDMA Cellular System.xe2x80x9d
Other devices such as the SeeGull(trademark) (available from Dynamic Telecommunications, Inc. of Germantown, Md.) also comprise a pn-scanner suited for micro cell analysis.
What is needed is a method and apparatus to modify the Ec/Io readings such that interferences from adjacent and coincident with the CDMA channel of interest can be removed from the measurements. This would allow the tester to obtain a reliable approximation of the base station""s transmission Ec/Io.
It is an object of the present invention to provide a simple method and apparatus for testing CDMA signal propagation and coverage.
It is another object of the present invention to provide a method and apparatus for calculating a corrected CDMA transmission signal to power that approximates the Ec/Io value of the CDMA transmission at the site of the transmitter.
These and other objectives of the present invention will become apparent to those skilled in the art from a review of the specification that follows.
A pn-scanner does a pilot scan of the CDMA channel of interest. The scan tests and measures energy per chip (Ec) and the total amount of power (Io). The ratio Ec/Io is an indicator of the quality and propagation of the signal. The present invention tests to see if an unacceptable Ec/Io ratio is due to interference from other transmissions within or close to the same transmission RF. A frequency scan performed close in time to the pn-scan is used to identify transmission signals that add strength to the Io value.
One embodiment of the present invention uses spectrum measurements on the signal received by the frequency scanner. Taking spectrum measurements and using pattern recognition analysis provides identification of discrete interference sources.
Interference component information including magnitude of the power, the transmission frequency and the type of signal is sent to a processor. In one embodiment, the processor is a digital signal processor (DSP), which may be integrated with either or both scanners. The present invention analyzes this information to adjust the Ec/Io for interference.
Depending on the kind and frequency offset from the nominal receive frequency of the identified spectral components, the individual interference items are treated in different ways to obtain the correcting factor for the value of the total power (Io) of the received signal. The goal is to correct the value of Io to obtain the value that characterizes only the power of the useful spread-spectrum signal exclusive of the interference power identified.
The processor computes a delta value of the Io due to interference sources. This delta value is subtracted from the total power as measured by the pn-scanner. The resultant is an adjusted Io (Ioi). The adjusted Ioi is used to calculate a modified Ec/Io ratio which approximates the signal to power ratio at the CDMA transmission site.
One embodiment of the present invention weights the individual interference components. Weighting is a function of the interference component""s frequency location relative to the CDMA channel of interest. For example, the adjacent-channel CDMA or other spread-spectrum signals will be entered in the correction expression with the weight determined during the design or calibration of the measurement scanner. The weight reflects the ratio of the power in the adjacent CDMA frequency channel that leaks into the CDMA channel of interest thus contributing to the growth of the total power (Io).
One embodiment applies weights to discrete interference components dependent on their position proximity to the edge of the CDMA channel of interest bandwidth. Those components well inside the bandwidth that is near the bandwidth center will have unity weight applied to the correction factor. The power of an interference component near the bandwidth edge will be weighted by the measured or theoretical value reflecting that component""s contributions to total power (Io). The interference components"" power near the bandwidth edges will have less than unity weighting applied. The processor individually calculates the weighting for each interference component.
One embodiment of the present invention reports the corrected Ec/Io ratio to the CDMA carrier. The sources of interference will also be reported. The CDMA carrier will be able to use this report for remedial action.